Diaryl ethers are an important class of industrial materials. Diphenyl oxide (DPO), for instance, has many uses, most notably as the major component of the eutectic mixture of DPO and biphenyl, which is the standard heat transfer fluid for the concentrating solar power (CSP) industry. With the current boom in CSP has come a tightening of the supply of DPO globally and questions surrounding the sustainability of the technology have arisen.
Diaryl ethers are currently manufactured commercially via two major routes: reaction of a haloaryl compound with an aryl alcohol; or gas-phase dehydration of an aryl alcohol. The first route, for example where chlorobenzene reacts with phenol in the presence of caustic and a copper catalyst, typically leads to less pure product and requires high pressure (5000 psig), uses an expensive alloy reactor and produces stoichiometric quantities of sodium chloride.
The second route, which is a more desirable approach, accounts for the largest volume of diaryl ethers produced but requires a very active and selective catalytic material. For instance, DPO can be manufactured by the gas-phase dehydration of phenol over a thorium oxide (thoria) catalyst (e.g., U.S. Pat. No. 5,925,798). A major drawback of thoria however is its radioactive nature, which makes its handling difficult and potentially costly. Furthermore, the supply of thoria globally has been largely unavailable in recent years putting at risk existing DPO manufacturers utilizing this technology. Additionally, other catalysts for the gas-phase dehydration of phenol, such as zeolite catalysts, titanium oxide, zirconium oxide and tungsten oxide, generally suffer from lower activity, significantly higher impurity content and fast catalyst deactivation.
With a chronic shortage of diaryl ethers such as DPO in sight and a pressing need to increase capacity, it has become crucial to develop alternate methods to produce such materials in a cost-effective and sustainable manner.
The problem addressed by this invention, therefore, is the provision of new catalysts and methods for manufacture of diaryl ethers from aryl alcohol compounds.